This invention relates generally to screens and more particularly concerns screens used for filtering undesirable particles from a liquid such as oil well drilling fluids.
A variety of woven cloth screens for use in removing particles from a liquid are designed to provide a tortuous path for the liquid. These woven cloths, including the typical weave, twill, dutch weave or twill dutch weave cloths, exhibit low fluid conductance characteristics, their very purpose being the formation of the tortuous flow path. Since the rate of flow is minimal, the filtering process is concomitantly slow. Furthermore, the screens need frequent cleaning to maintain any degree of functionality.
Other screens have been developed which do provide an open surface area and therefore permit direct or nontortuous flow through the screen. While these screens do provide better conductance characteristics, the fluid conductance is generally limited by the permissible ratio of length to width in the interstices between the screen filaments and the fineness of the filaments. As the spacing between filaments increases, the likelihood of deformation of the filaments from the parallel increases, with result that larger than desired particles can pass through the screen. In order to maintain a workable relationship, the size of the rectangular interstices in these screens is generally minimal and the length to width ratio is generally less than three unless coarse, stiff filaments are used. Sometimes higher ratios can be achieved by bonding together the crossing filaments of the screen. However, bonding is a complex and costly process with negative side effects. Bonding may be accomplished by coating the filaments with a bonding material, which in turn increases the diameter of the filaments and further reduces the fluid conductance of the screen. Bonding is not a routine practice.
It is, therefore, an object of this invention to provide a screen for filtering undesirable particles from a liquid having fluid conductance characteristics superior to those presently known. It is a further object of this invention to provide such a screen having length to width or aspect ratios of the interstices therein in excess of ratios presently in use for relatively thin filaments. A further object of this invention is to provide a screen which provides improved fluid conductance characteristics without the need for coating the filaments or bonding the filaments of the screen together.